|Comfort above the clouds
Comfort above the clouds This article is published in AA&A Magazine
By Rosemarie John - Despite various innovations to improve passenger comfort in 55 years of commercial jet aviation, very few individuals would describe any flight as truly relaxing and resulting in one feeling fresh as a daisy when disembarking at the destination airport. So, what gives with the new generation of jetliners – the Airbus A380 and Boeing Dreamliner 787 – which aim to bring passenger comfort even closer to this subjective standard by increasing cabin pressures to a hitherto-unheard of 5,000 feet altitudes?
Comfort above the clouds
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The seasoned traveler knows all too well how daunting air travel can be. Dry, cramped conditions in an airplane make the flying experience far from comfortable and oftentimes, passengers could experience flu-like symptoms shortly after they’ve boarded the plane.
The slight headaches, light-headedness, sore throats, coughing, dry lips and dry or watery eyes are in fact due to the lack of fresh air that only an aircraft cabin on modern jets can so effectively create – since BOAC inaugurated the world's first commercial jet service on 2 May 1952.
Such environmental conditions are due to the airplane’s cabin being pressurized at current international requirements set for an altitude of 8,000 feet – resulting in passengers breathing in roughly 25% less oxygen compared to sea levels.
The culprit here is an increased level of carbon dioxide in the bloodstream, exacerbated by conditions of limited air supplies shared by too many people. Such an artificially-created altitude pressure in aircraft cabins also results in passengers feeling dried out at the end of a long airplane flight due to the lower humidity levels.
The new passenger jets coming on-stream in the next few months promise to drastically reduce this discomfort by keeping cabin pressures at altitudes ranging between 5,000 and 6,000 feet. These altitudes are more reminiscent of an elegant resort rather than a lofty mountain spire. But is there a real difference the passenger can look forward to in terms of comfort levels?
The complex interplay between three main factors – oxygen, carbon dioxide and humidity levels – that affect passenger comfort is the guideline used in the setting of cabin altitudes.
Starting with oxygen, its air composition ration remains at roughly 21% regardless of altitude. However, as overall air pressure drops with increasing altitudes, the number of oxygen molecules per breath is reduced. Hence, to properly oxygenate the body, one’s breathing rate (even while at rest) has to increase. Since the amount of oxygen required for activity is the same, the body must adjust to having less oxygen.
The usual side effect, which typically starts once you pass the 3,000 feet altitude, is reduced alertness and a growing sense of lethargy unless one’s body becomes used to a more rapid breathing rate. Once the 8,000 feet altitude is passed, these side effects become more pronounced.
The new lower cabin altitude pressurized at 5,000 to 6,000 feet aims to reduce this sense of lethargy and increase alertness with higher oxygen pressures – which would theoretically result in passengers feeling more refreshed as the pressure differential would not be too far removed from what one would usually be acclimatized to.
Even so, there is the second factor of carbon dioxide levels which also have to be considered.
At higher altitudes, oxygen saturation in the bloodstream drops from about 98% (at sea levels) to about 90% once one passes 6,000 feet. The difference results in a higher saturation of carbon dioxide in the bloodstream.
These changes in gas saturation levels represent a virtually imperceptible change for most travelers as it creeps up on a passenger like a mild hangover with the most common signs being anything from shortness of breath, nausea, loss of appetite, mild headache, and fatigue.
But those who are worse affected may experience increased shortness of breath, a dry cough, wheezing, increased weakness and rapid pulse.
By keeping cabin altitude pressures at below 6,000 feet, new aircraft aim to reduce or completely eradicate these symptoms – thus increasing overall passenger comfort.
The third factor of humidity is actually less important than one might perceive for passenger comfort as it is actually more dependent on the relative cabin temperature than on altitude pressures.
By lowering cabin temperatures, passenger activity is lowered (since there isn’t really all that much space available for any vigorous exercising). This results in overall reduction in the need for higher oxygen saturation in passengers’ bloodstreams – which ties in well with the set cabin altitudes.
Since the cabin air is on the dry side with lower temperatures, the passenger starts to feel parched as the flight wears on and begins to feel more lethargic unless one gets hydrated regularly. But, with changes in the new planes reducing this need, it is possible to raise cabin temperatures and overall humidity levels as well. The more moist air results in passengers feeling less dried out and hence feeling fresher at the end of the trip.
In theory, the lower cabin altitude will help to moderate almost every ill-effect of long-haul flying, from dehydration to jet lag. However, it should be noted that some people might still experience symptoms of altitude sickness despite lower cabin pressure.
“It is extremely difficult to quantify the effect on passenger comfort level in real conditions as your overall feeling of well-being depends on so many inter-related factors such as temperature, air flow, air filtration and humidity as well as cabin pressure and its rate of change,” said Airbus aircraft interiors marketing head Bob Lange.
“Although cabin altitude in the A380 can be up to 1,000 feet lower than in some older wide-bodies, we pay attention to all of the parameters that affect cabin air quality in order to improve passenger well being in flight.”
“The A380 at a typical cruising altitude of 35,000 feet has the cabin air pressurized to an equivalent of 5,000 feet altitude. When the aircraft climbs to 39,000 feet towards the end of the flight the cabin altitude will gently rise to around 6,000 feet. However, great attention has been paid to limit the rate of change of cabin pressure because many of the symptoms described below are linked to the body’s speed of adjustment to the changes,” said Lange.
Why has it taken so long for aircraft manufacturers to offer these small but significant improvements in passenger comfort levels?
The biggest issue just happens to be relative air humidity. As can often be seen in cabins in-flight, condensation forms on cold surfaces – and this can cause mold, corrosion and moisture related deterioration in an aircraft’s fuselage.
There is also the matter of the fuselage composition itself, which has to withstand high pressure differentials within and without the aircraft.
The fuselage of older planes are made of aluminum and as it is pressurized and depressurized, the metal skin of the airplane expands and contracts, resulting in metal fatigue and also corrosion if humidity is increased.
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In aluminum airplanes, cabin humidity is kept low intentionally to prevent condensation on the airplane’s interior surfaces. Condensed moisture can build up in insulation materials and eventually contribute to corrosion of the airplane’s metal structure.
“Changing cabin pressures in existing aircraft is not that simple because of their aluminum structures. Airplanes are designed to withstand the continuous pressurization/depressurization cycles that occurs in takeoffs and landings throughout their service life,” said Boeing environmental performance director Jeanne Yu.
“To lower the maximum cruise cabin altitude, the pressure difference must be increased between the interior and exterior of the airplane. This change would fatigue (wear out) the structure more rapidly and impact the airplane's service life in an uncertain way, perhaps even compromising the design integrity of the airplane,”
“Differential pressure is the difference between the pressure of the air inside the aircraft cabin and that of the air outside at flight altitude,” said Lange. “It is expressed in psi (pounds per square inch), which can most easily be imagined as a force exerted across a surface area. The greater that force, the stronger the surface (in this case the aircraft fuselage) needs to be to withstand it.”
Comfort above the clouds
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Aircrafts have a positive pressure relief valve in the event of excessive pressure in the cabin to protect the aircraft structure from excessive loading. The maximum pressure differential between the cabin and the outside ambient air is between 7.5 and 9psi.
“On a B747-300/200/100, the flight engineer would readjust the cabin altitude to the en-route clearance which includes the initial cruising altitude provided by the ATC,” said retired flight engineer John Paul who has been flying for 39 years with various airlines.
“Having adjusted the cruising altitude, the automatic pressurization controller will automatically control the cabin altitude to maintain a cabin differential pressure of 8.5psi.”
Pressurization failures rarely take place. Should they occur, it is usually due to leaks through door seals, a pneumatic leak or an electronic pressurization controller problem – easily correctable by cockpit crew when a rapid depressurization checklist is carried out.
“Oxygen helps avert the effects of depressurization at altitude. The oxygen from these masks usually last for about 10 minutes, giving sufficient time for the pilot to descend the aircraft from the cruising altitude to about 10,000 to 15,000 feet,” said Malaysia Airlines medical services senior manager Dr Daljit S Parmar.
“Cabin crew are trained on the usage and applications of various oxygen systems used onboard the aircraft and are also trained to handle passengers who may react differently to the effect of hypoxia occurring from rapid depressurization,” he added.
However, should an aircraft suffer a pressurization failure above 10,000 feet due to worse causes, the pilot would immediately place the plane in an emergency descent and activate oxygen masks for everyone on board. Passenger oxygen masks are automatically deployed if the cabin altitude is above 14,000 feet.
With the advancement of technology and having the fuselage made out of composites rather than aluminum, it’s possible to increase the humidity without corroding the airplane over time. Composites used in the new aircraft are not subject to the same fatigue conditions that limit the amount of pressure cycles that can be applied to an aluminum airplane.
Looking at the Boeing 787 Dreamliner’s composite structure – which does not corrode when exposed to moisture plus insulation designed to resist the buildup of moisture – the engineers could then incorporate an air conditioning system that retains a comfortable level of humidity throughout the flight.
The first aircraft with cabin pressurization (though restricted to crew areas) was the B-29 Superfortress. Post-war piston airliners such as the Lockheed Costellation brought the technology to civilian service and as jet airliners were always designed for high-altitude operation, every jetliner features the technology.
Cabin pressurization is the active pumping of air into an aircraft cabin to increase the air pressure within the cabin. The cabins of most civil transport are pressurized with air supplied by the engines.
Bleed air extracted from the engines is compressively heated and extracted at approximately 200°C and then cooled by passing it through a heat exchanger and air cycle machine. An environmental control system (ECS) using air provided by compressors or bleed air pressurizes commercial aircrafts.
Today’s aircraft have a dual channel electronic controller for maintaining pressurization along with a manual back-up system. These systems maintain cabin air pressure equivalent to 8,000 feet or less, even during flight at altitudes above 43,000 feet.
In passenger-carrying aircraft, the degree of pressurization of the cabin is determined by the requirements to prevent significant hypoxia (shortage of oxygen in one’s blood, and hence the body) at altitude and damage to the middle ear on descent.
The flow of air through the cabin is determined principally by the requirements for ventilation and thermal comfort. The cabin air outlet valves control the differential pressure between the pressure cabin and the environment.
Cabin air on older planes has very low humidity levels (15-20% relative humidity), due to very dry air being brought in from outside at high altitude. The air outside the plane is very cold and thus has a very low absolute humidity, which translates into a very low humidity level when warmed.
Low humidity in the cabin is caused by the frequent renewal of cabin air with outside air. Since the outside temperature at typical cruising altitudes is very low (-5oC to -25oC), it contains little moisture. It is this very dry air that is supplied to the cabin.
During flight, the relative humidity in the cabin ranges from approximately 5% to 35%, with an average of 15% to 20%. This is similar to the dry summer climate of the southwestern United States or typical wintertime indoor levels.
In addition, for older generation airplanes flying currently, cabin air comes from the inside of the jet engine while in newer airplanes, the cabin air system will be vented directly from the outside through dedicated inlets on each side of the plane's belly and won't pass through the engines.
These airplanes are powered by high-bypass-ratio fan engines which are much quieter, much cleaner burning, more powerful and much more efficient.
At the front end of this engine type is a large-diameter fan, which is powered by the core. The fan moves a large volume of air past the core rather than through it, and actually generates most of the thrust.
By providing the cabin with a mixture of about 50% outside air taken from the compressor and 50% recirculated air, a balance has been achieved that maintains a high level of cabin air quality in turn enhancing passenger comfort.
Our body’s reaction
One reason for feeling dried out is that the air conditioning system aboard conventional airplanes gradually replaces cabin air with the much drier air from the high-altitude atmosphere outside. The cabin atmosphere, which starts out with the same humidity as the city where you boarded, becomes increasingly dry as the flight progresses.
Exposure to low humidity environment without sufficient fluid intake will dehydrate the body through perspiration and respiratory water loss. Dehydration can lead to headaches, tiredness and fatigue. In addition, low humidity can cause drying of the nose, throat and eyes, and it can irritate contact lens wearers.
Subsequently, pressurization is required when an aircraft reaches high altitudes, because the natural atmospheric pressure is too low to allow people to absorb sufficient oxygen, leading to altitude sickness.
Altitude diagram Altitude diagram
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Altitude sickness is a reaction to the lower amounts of oxygen available at high altitudes. As an airplane pressurizes and decompresses, some passengers will experience discomfort as trapped gasses within their bodies expand or contract in response to the changing cabin pressure.
The most common problems occur with gas trapped in the gastrointestinal tract, the middle ear and the paranasal sinuses along with headaches and nausea.
Effects of cabin pressurization on the human body:
• Ear and paranasal sinuses – One needs to adjust to the pressurized cabin air from the beginning. Rapid changes in air pressure cause the air pocket inside the ear to expand during takeoff and contract during descent, stretching the eardrum. To equalize pressure, air must enter or escape through the Eustachian tube. Air trapped in the sinuses due to a cold or allergy may not cause a problem as the cabin altitude rises, but the pain can be unbearable when a rapid correction is made to decrease altitude and restore pressure.
• Tooth – Anyone with intestinal gas or gas trapped in an infected tooth may also experience Barodontalgia, a toothache provoked by exposure to changing atmospheric pressure. Air trapped in dental work can cause excruciating symptoms as pressure decreases and the air expands.
• Pneumothorax – Anyone who has suffered a pneumothorax is recommended not to fly for at least 1 month and should obtain an x-ray prior to traveling.
In some individuals, particularly those with heart or lung disease, symptoms may begin as low as 6,000 feet above sea level, although most passengers can tolerate altitudes of around 8,000 feet without ill effect.
• For someone with lung disease, a loss of a few percentage points in oxygen saturation could trigger shortness of breath. A person with heart disease may suffer chest pain, or have an increased risk of a heart attack or irregular heart rhythm. It's also possible that significant drops in oxygen levels could contribute to deep vein thrombosis (DVT) - blood clots in the legs that some passengers develop during long-haul flights. Besides the effects it can have on the chronically ill, oxygen deprivation can create some less serious problems during and after a flight, including physical and mental fatigue, headache and digestive problems.
While the chances of extreme unplanned cabin pressurization failure might be relatively small on any given flight, it is still wise to take the necessary precautions before traveling. Some solutions to prevent unsavory effects:
• Do not fly when having a cold; it may make inflation of the middle ear much more difficult.
• Pay attention to the flight attendants and listen to safety briefings, even if you've heard them many times.
• If your ears hurt when you fly, try taking a decongestant medicine before you get on the plane. You can also swallow often and chew gum during the flight. Babies can suck on bottles or a pacifier during the flight.
• Drink plenty of fluids (preferably water) before, during and after your flight. Not only will you feel better but also keeping up your fluid reserves also helps your body ward off a whole other host of maladies. Staving off dehydration will also decrease your risk of getting jet lag.
• If need be, slap on some moisturizer and lip balm to combat the dry air in the flight cabin.
• If you wear glasses, don’t fly with contacts - don your specs instead. Contacts will only dry out and further irritate itchy or burning eyes.
• If your budget allows, fly in first or business class, where fewer people share air space and seating is much more spacious. If you’re flying coach, request a seat in an emergency exit row - the roomiest of all rows.
• Don’t leave home without your inhaler if you have asthma.
• In-flight symptoms such as breathlessness, chest pain or confusion may signal that a person has dropping oxygen levels. In such cases, passengers with heart or lung disease can ask the crew for oxygen. Healthy people may need only to drink some water, as dehydration compounds the effects of oxygen loss. Avoiding alcohol and sleeping pills may also help.
• It is suggested that before taking a flight, people with heart or lung disease have their doctors measure their oxygen saturation. If it is already low, then patients will know it could drop to problematic levels and they could tell the airline they will need oxygen during the flight.
• If you’re on a connecting flight and have sufficient time, try to get as much fresh air as you can between connections.
• Avoid smoky bars when you reach your destination or while you wait for a connecting flight
• Clear your head with a hot, steamy shower after you land.
• It's dangerous to fly immediately after scuba diving. You'll need to wait 12 to 24 hours after diving. Ask your doctor or diving authorities for guidelines on flying after scuba diving.